Yes, of course.

However, this requires the use of logic in the microflow. You can, for example, set an OnChange action or when saving, which will call the microflow. 

The parameter of such a microflow should be the entity to which the check applies.

You then need to create logic that validates (e.g. if value = ‘A’) and for an incorrect value, using Decision (yellow rhombus) use the Validation Feedback toolbox.

You indicate which entity and which parameter you want the information to refer to. In the event of an incorrect value, i.e. an incorrect value under the combo box, a message will appear with the text you set in Validation Feedback 

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Hi Sergio González Iglesias,

You can add validation to a combo box. At the microflow level, you can incorporate a decision node, and based on that decision, you can pass the validation feedback message. To display the asterisk (*) indicating a required field, you need to add a custom class in the properties pane.